Bump minimum python version to 3.7.0 (#2394)

This is in anticipation of #2246, although I've written that PR to not break on 3.6, the feature itself is not usable on 3.6. So I think the best way forward is to simply require python 3.7. This also allows devs and cog creators to utilise all of the new features in 3.7, and it also updates the docs so all operating systems will have 3.7 installed.

Signed-off-by: Toby <tobyharradine@gmail.com>

tox.ini

@@ -5,7 +5,6 @@
 
 [tox]
 envlist =
-    py36
     py37
     docs
     style
@@ -24,7 +23,7 @@ commands =
 description = Attempt to build docs with sphinx-build
 whitelist_externals =
     sphinx-build
-basepython = python3.6
+basepython = python3.7
 extras = docs, mongo
 commands =
     sphinx-build -d "{toxworkdir}/docs_doctree" docs "{toxworkdir}/docs_out/html" -W -bhtml
@@ -39,7 +38,7 @@ setenv =
     # This is just for Windows
     # Prioritise make.bat over any make.exe which might be on PATH
     PATHEXT=.BAT;.EXE
-basepython = python3.6
+basepython = python3.7
 extras = style
 commands =
     make stylecheck